Plants Used By People

Outside of formal scientific contexts, the term "plant" implies an association with certain traits, such as multicellularity, cellulose, and photosynthesis. Many of the classification controversies involve organisms that are rarely encountered and are of minimal apparent economic significance, but are crucial in developing an understanding of the evolution of modern flora.
• Algae
Most algae are no longer classified within the Kingdom Plantae. The algae comprise several different groups of organisms that produce energy through photosynthesis, each of which arose independently from separate non-photosynthetic ancestors. Most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. Each of these algal groups also includes various microscopic and single-celled organisms.
The two groups of green algae are the closest relatives of land plants (embryophytes). The first of these groups is the Charophyta (desmids and stoneworts), from which the embryophytes developed. The sister group to the combined embryophytes and charophytes is the other group of green algae, Chlorophyta, and this more inclusive group is collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing cellulose, have chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae.